1. Field of the Invention
This invention relates to an A/D (analog to digital) converter of an image signal in an imaging device or the like.
2. Description of the Prior Art
Examples of known A-D converters are shown in U.S. Pat. Nos. 4,179,687 and 4,216,465. The converters shown in those patents incorporate variously quantized steps of signal conversion (see FIG. 7 and col. 8, lines 20-38 of U.S. Pat. No. 4,179,687 and FIGS. 2, 3, 5 and 8 and col. 4, lines 3-55, col. 5, line 67 to col. 6 line 2, col. 6 lines 37-63 and col. 8, line 50 to col. 9, line 10 of U.S. Pat. No. 4,216,465).
There have been many well-known A/D converters of image signals. In accordance with a recent tendency of providing a wider band for signals, there have been practically utilized A/D converters in which A/D (analog to digital) conversion speed is improved up to several tens of MHz (megahertz) and quantization is performed in 8 bit increments.
However, in A/D conversion of image signals in an imaging device, the conventional quantization bit number is insufficient due to the following reasons.
(1) The dynamic range of image signals is several times larger than that of usual television signals.
(2) When gamma correction is performed after A/D conversion, quantization noises at low-brightness portions are increased at least less than three times.
(3) When white balance is performed after A/D conversion, an incease in dynamic range of at least twice is further required.
(4) When AGC (automatic gain control) is performed after A/D conversion, an increase in dynamic range of several times is further required.
Since the quantization bit number is insufficient as described above, there has hitherto been a need for performing analog signal processing, such as white compression, gamma correction, white balance and the like, before A/D conversion. This causes an increase of circuit scale, and the effect of digital signal processing is offset.
An increase of quantization bit number naturally causes a tremendous increase of circuit scale and insufficiency in A/D conversion speed, and so is very difficult in the present A/D conversion and LSI (large scale integrated circuits) techniques.